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Query Parameters and String Validations

FastAPI allows you to declare additional information and validation for your parameters.

Let's take this application as example:

=== "Python 3.6 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial001.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial001_py310.py!}
```

The query parameter q is of type Union[str, None] (or str | None in Python 3.10), that means that it's of type str but could also be None, and indeed, the default value is None, so FastAPI will know it's not required.

!!! note FastAPI will know that the value of q is not required because of the default value = None.

The `Union` in `Union[str, None]` will allow your editor to give you better support and detect errors.

Additional validation

We are going to enforce that even though q is optional, whenever it is provided, its length doesn't exceed 50 characters.

Import Query

To achieve that, first import Query from fastapi:

=== "Python 3.6 and above"

```Python hl_lines="3"
{!> ../../../docs_src/query_params_str_validations/tutorial002.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="1"
{!> ../../../docs_src/query_params_str_validations/tutorial002_py310.py!}
```

Use Query as the default value

And now use it as the default value of your parameter, setting the parameter max_length to 50:

=== "Python 3.6 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial002.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial002_py310.py!}
```

As we have to replace the default value None in the function with Query(), we can now set the default value with the parameter Query(default=None), it serves the same purpose of defining that default value.

So:

q: Union[str, None] = Query(default=None)

...makes the parameter optional, the same as:

q: Union[str, None] = None

And in Python 3.10 and above:

q: str | None = Query(default=None)

...makes the parameter optional, the same as:

q: str | None = None

But it declares it explicitly as being a query parameter.

!!! info Have in mind that the most important part to make a parameter optional is the part:

```Python
= None
```

or the:

```Python
= Query(default=None)
```

as it will use that `None` as the default value, and that way make the parameter **not required**.

The `Union[str, None]` part allows your editor to provide better support, but it is not what tells FastAPI that this parameter is not required.

Then, we can pass more parameters to Query. In this case, the max_length parameter that applies to strings:

q: Union[str, None] = Query(default=None, max_length=50)

This will validate the data, show a clear error when the data is not valid, and document the parameter in the OpenAPI schema path operation.

Add more validations

You can also add a parameter min_length:

=== "Python 3.6 and above"

```Python hl_lines="10"
{!> ../../../docs_src/query_params_str_validations/tutorial003.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial003_py310.py!}
```

Add regular expressions

You can define a regular expression that the parameter should match:

=== "Python 3.6 and above"

```Python hl_lines="11"
{!> ../../../docs_src/query_params_str_validations/tutorial004.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial004_py310.py!}
```

This specific regular expression checks that the received parameter value:

  • ^: starts with the following characters, doesn't have characters before.
  • fixedquery: has the exact value fixedquery.
  • $: ends there, doesn't have any more characters after fixedquery.

If you feel lost with all these "regular expression" ideas, don't worry. They are a hard topic for many people. You can still do a lot of stuff without needing regular expressions yet.

But whenever you need them and go and learn them, know that you can already use them directly in FastAPI.

Default values

The same way that you can pass None as the value for the default parameter, you can pass other values.

Let's say that you want to declare the q query parameter to have a min_length of 3, and to have a default value of "fixedquery":

{!../../../docs_src/query_params_str_validations/tutorial005.py!}

!!! note Having a default value also makes the parameter optional.

Make it required

When we don't need to declare more validations or metadata, we can make the q query parameter required just by not declaring a default value, like:

q: str

instead of:

q: Union[str, None] = None

But we are now declaring it with Query, for example like:

q: Union[str, None] = Query(default=None, min_length=3)

So, when you need to declare a value as required while using Query, you can simply not declare a default value:

{!../../../docs_src/query_params_str_validations/tutorial006.py!}

Required with Ellipsis (...)

There's an alternative way to explicitly declare that a value is required. You can set the default parameter to the literal value ...:

{!../../../docs_src/query_params_str_validations/tutorial006b.py!}

!!! info If you hadn't seen that ... before: it is a special single value, it is part of Python and is called "Ellipsis".

It is used by Pydantic and FastAPI to explicitly declare that a value is required.

This will let FastAPI know that this parameter is required.

Required with None

You can declare that a parameter can accept None, but that it's still required. This would force clients to send a value, even if the value is None.

To do that, you can declare that None is a valid type but still use default=...:

=== "Python 3.6 and above"

```Python hl_lines="8"
{!> ../../../docs_src/query_params_str_validations/tutorial006c.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial006c_py310.py!}
```

!!! tip Pydantic, which is what powers all the data validation and serialization in FastAPI, has a special behavior when you use Optional or Union[Something, None] without a default value, you can read more about it in the Pydantic docs about Required Optional fields.

Use Pydantic's Required instead of Ellipsis (...)

If you feel uncomfortable using ..., you can also import and use Required from Pydantic:

{!../../../docs_src/query_params_str_validations/tutorial006d.py!}

!!! tip Remember that in most of the cases, when something is required, you can simply omit the default parameter, so you normally don't have to use ... nor Required.

Query parameter list / multiple values

When you define a query parameter explicitly with Query you can also declare it to receive a list of values, or said in other way, to receive multiple values.

For example, to declare a query parameter q that can appear multiple times in the URL, you can write:

=== "Python 3.6 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial011.py!}
```

=== "Python 3.9 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial011_py39.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial011_py310.py!}
```

Then, with a URL like:

http://localhost:8000/items/?q=foo&q=bar

you would receive the multiple q query parameters' values (foo and bar) in a Python list inside your path operation function, in the function parameter q.

So, the response to that URL would be:

{
  "q": [
    "foo",
    "bar"
  ]
}

!!! tip To declare a query parameter with a type of list, like in the example above, you need to explicitly use Query, otherwise it would be interpreted as a request body.

The interactive API docs will update accordingly, to allow multiple values:

Query parameter list / multiple values with defaults

And you can also define a default list of values if none are provided:

=== "Python 3.6 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial012.py!}
```

=== "Python 3.9 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial012_py39.py!}
```

If you go to:

http://localhost:8000/items/

the default of q will be: ["foo", "bar"] and your response will be:

{
  "q": [
    "foo",
    "bar"
  ]
}

Using list

You can also use list directly instead of List[str] (or list[str] in Python 3.9+):

{!../../../docs_src/query_params_str_validations/tutorial013.py!}

!!! note Have in mind that in this case, FastAPI won't check the contents of the list.

For example, `List[int]` would check (and document) that the contents of the list are integers. But `list` alone wouldn't.

Declare more metadata

You can add more information about the parameter.

That information will be included in the generated OpenAPI and used by the documentation user interfaces and external tools.

!!! note Have in mind that different tools might have different levels of OpenAPI support.

Some of them might not show all the extra information declared yet, although in most of the cases, the missing feature is already planned for development.

You can add a title:

=== "Python 3.6 and above"

```Python hl_lines="10"
{!> ../../../docs_src/query_params_str_validations/tutorial007.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="8"
{!> ../../../docs_src/query_params_str_validations/tutorial007_py310.py!}
```

And a description:

=== "Python 3.6 and above"

```Python hl_lines="13"
{!> ../../../docs_src/query_params_str_validations/tutorial008.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="12"
{!> ../../../docs_src/query_params_str_validations/tutorial008_py310.py!}
```

Alias parameters

Imagine that you want the parameter to be item-query.

Like in:

http://127.0.0.1:8000/items/?item-query=foobaritems

But item-query is not a valid Python variable name.

The closest would be item_query.

But you still need it to be exactly item-query...

Then you can declare an alias, and that alias is what will be used to find the parameter value:

=== "Python 3.6 and above"

```Python hl_lines="9"
{!> ../../../docs_src/query_params_str_validations/tutorial009.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="7"
{!> ../../../docs_src/query_params_str_validations/tutorial009_py310.py!}
```

Deprecating parameters

Now let's say you don't like this parameter anymore.

You have to leave it there a while because there are clients using it, but you want the docs to clearly show it as deprecated.

Then pass the parameter deprecated=True to Query:

=== "Python 3.6 and above"

```Python hl_lines="18"
{!> ../../../docs_src/query_params_str_validations/tutorial010.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="17"
{!> ../../../docs_src/query_params_str_validations/tutorial010_py310.py!}
```

The docs will show it like this:

Exclude from OpenAPI

To exclude a query parameter from the generated OpenAPI schema (and thus, from the automatic documentation systems), set the parameter include_in_schema of Query to False:

=== "Python 3.6 and above"

```Python hl_lines="10"
{!> ../../../docs_src/query_params_str_validations/tutorial014.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="8"
{!> ../../../docs_src/query_params_str_validations/tutorial014_py310.py!}
```

Recap

You can declare additional validations and metadata for your parameters.

Generic validations and metadata:

  • alias
  • title
  • description
  • deprecated

Validations specific for strings:

  • min_length
  • max_length
  • regex

In these examples you saw how to declare validations for str values.

See the next chapters to see how to declare validations for other types, like numbers.